Passing a coroutine to AbstractEventLoop.call_later


The following is the code I’m trying to get working:

>>> import asyncio
>>> async def foo(loop, iv):
...     await asyncio.sleep(1, loop=loop)
...     print(f'done: {iv}')
>>> loop = asyncio.get_event_loop()
>>> loop.call_later(2, foo, loop, 10)
<TimerHandle when=36395.554089349 foo(<_UnixSelecto...e debug=False>, 10) at <input>:1>
>>> loop.run_forever()

(Python 3.6)

Basically the foo() function has some chained async calls, so this method has to be async as there is a need to await for the chained calls. However this method is triggered after a delay, and when one runs this code, the following problem occurs:

/usr/lib64/python3.6/asyncio/ RuntimeWarning: coroutine
‘foo’ was never awaited self._callback(*self._args)

What is the correct way to handle this async call in the call_later?

Asked By: Nim



call_later() only supports callbacks (regular functions); you simply can’t pass in a coroutine.

If you want to delay a coroutine, you have two options; either delay the coroutine by having it sleep at the start, or call asyncio.create_task() from call_later(), which does take a coroutine and schedules it to run.

use asyncio.sleep() at the start of the routine, you can have the loop execute it directly:

async def foo(iv):
    # delay start of the work
    await asyncio.sleep(2)

    # rest of your coroutine

You could easily use a wrapper coroutine to do this:

async def await_coro_later(delay, coro, *args, **kwargs):
    await asyncio.sleep(delay)
    await coro(*args, **kwargs)

If you use asyncio.create_task() (or, for Python 3.6 or older, asyncio.ensure_future(), you can pass that to call_later():

# create a task for foo(10) later
loop.call_later(2, asyncio.create_task, foo(10))

Demo of either technique:

>>> import asyncio
>>> async def foo(iv, start):
...     await asyncio.sleep(1)
...     offset = asyncio.get_running_loop().time() - start
...     print(f'done ({offset:.3f}s): {iv}')
>>> async def await_coro_later(delay, coro, *args, **kwargs):
...     await asyncio.sleep(delay)
...     await coro(*args, **kwargs)
>>> async def demo():
...     loop = asyncio.get_running_loop()
...     start = loop.time()
...     loop.call_later(2, asyncio.create_task, foo('cb_to_create_task', start))
...     await await_coro_later(5, foo, 'coro_later', start)
done (3.004s): cb_to_create_task
done (6.006s): coro_later
Answered By: Martijn Pieters

Small addition.

It seems that better way is to use lambda.
Instead of use:
loop.call_later(2, asyncio.create_task, foo('cb_to_create_task', start))
loop.call_later(2, lambda: asyncio.create_task(foo('cb_to_create_task', start)))

It’s avoid "RuntimeWarning: coroutine ‘ResultCursor.finalize’ was never awaited" if timeout cancellation is used and asyncio debugging is enabled.

Answered By: x7273